Prior to this invention, conductive paste has been fabricated from silver particles, an inorganic bonding component, and an organic binding component. Typically, such a paste contains, by weight, 60 to 70% silver, 5 to 10% glass frit and 20 to 35% of a mixture of various solvents, plasticizers and resins. This paste has been applied to a substrate, for example, a ceramic capacitor, and the substrate and paste fired to form a component comprising the substrate and a fired-on electroconductive body which provided an electrically conductive connection. Frequently, these components have been coated with solder for ready integration into a circuit at a later date.
The foregoing prior paste compositions and electroconductive bodies are, in some applications, unsatisfactory because, among other reasons, after contact with the solder some of the silver in the electroconductive body migrates or leaches out of the body into the solder. This phenomenon is known in the art as "de-wetting" and often causes a significant decrease in the conductivity of the electroconductive body, as well as impairing its adhesion to the substrate surface.
Another disadvantage of these precursor paste compositions and electroconductive bodies is that fabrication of the electroconductive body must be performed by heating the paste at high temperatures which may damage the substrate to which a paste has been applied.
Attempts have been made to find satisfactory substitutes for the above-mentioned pastes and electroconductive bodies. It has been suggested that a conductive paste may be fired to produce an electroconductive body containing micron-sized glass spheres coated with a noble metal, such as palladium or platinum, or an alloy, for example of palladium, gold and silver, embedded in a matrix of glassy dielectric material having a fusion temperature lower than the softening temperature of the glass spheres. An electroconductive body consisting of particles of alumina coated by palladium, particles of alumina coated by palladium oxide and particles of silver embedded in a glassy matrix has also be suggested. Although electroconductive bodies of this character may be said to exhibit a somewhat increased resistance to de-wetting, the use of metals such as palladium and gold entails substantial expense in the production of the electroconductive body, and its precursor conductive paste. Also, disadvantages attendant to fabrication at high firing temperatures are not avoided with these substitutes.
It has also been suggested that a conductive paste comprising an organic resin binder and a particulated electrically conductive metal-containing material, for example silver-coated glass spheres may be treated to form a conductive coating. Other electrically conductive metals are also suggested. However, an electroconductive body with better electrical and other properties, at comparable or lesser expense, would be advantageous.